1. Field of the Invention
This invention relates generally to systems for drilling boreholes for the production of hydrocarbons from subsurface formations and more particularly to a closed-loop drilling system which includes a number of devices and sensors for determining the operating condition of the drilling assembly, including the drill bit, a number of formation evaluation devices and sensors for determining the nature and condition of the formation through which the borehole is being drilled and processors for computing certain operating parameters downhole that are communicated to a surface system that displays dysfunctions relating to the downhole operating conditions and provides recommended action for the driller to take to alleviate such dysfunctions so as to optimize drilling of the boreholes. This invention also provides a closed-loop interactive system that simulates downhole drilling conditions and determines drilling dysfunctions for a given well profile, bottom hole assembly, and the values of surface controlled drilling parameters and the corrective action which will alleviate such dysfunctions.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as logging-while-drilling ("LWD") tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Pressurized drilling fluid (commonly known as the "mud" or "drilling mud") is pumped into the drill pipe to rotate the drill motor and to provide lubrication to various members of the drill string including the drill bit. The drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes. The drill bit is typically coupled to a bearing assembly having a drive shaft which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the radial and axial forces of the drill bit.
Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations. The drilling operator typically controls the surface-controlled drilling parameters, such as the weight on bit, drilling fluid flow through the drill pipe, the drill string rotational speed (r.p.m of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid to optimize the drilling operations. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to optimize the drilling operations. For drilling a borehole in a virgin region, the operator typically has seismic survey plots which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator also has information about the previously drilled boreholes in the same formation. Additionally, various downhole sensors and associated electronic circuitry deployed in the BHA continually provide information to the operator about certain downhole operating conditions, condition of various elements of the drill string and information about the formation through which the borehole is being drilled.
Typically, the information provided to the operator during drilling includes: (a) borehole pressure and temperature; (b) drilling parameters, such as WOB, rotational speed of the drill bit and/ or the drill string, and the drilling fluid flow rate. In some cases, the drilling operator also is provided selected information about the bottomhole assembly condition (parameters), such as torque, mud motor differential pressure, torque, bit bounce and whirl etc.
The downhole sensor data is typically processed downhole to some extent and telemetered uphole by electromagnetic signal transmission devices or by transmitting pressure pulses through the circulating drilling fluid. Mud-pulse telemetry, however, is more commonly used. Such a system is capable of transmitting only a few (1-4) bits of information per second. Due to such a low transmission rate, the trend in the industry has been to attempt to process greater amounts of data downhole and transmit selected computed results or "answers" uphole for use by the driller for controlling the drilling operations.
Although the quality and type of the information transmitted uphole has greatly improved since the use of microprocessors downhole, the current systems do not provide to the operator information about dysfunctions relating to at least the critical drill string parameters in readily usable form nor do they determine what actions the operator should take during the drilling operation to reduce or prevent the occurrence of such dysfunctions so that the operator can optimize the drilling operations and improve the operating life of the bottomhole assembly. It is, therefore, desirable to have a drilling system which provides the operator simple visual indication of the severity of at least certain critical drilling parameters and the actions the operator should take to change the surface-controlled parameters to improve the drilling efficiency.
A serious concern during drilling is the high failure rate of bottom hole assembly and excessive drill bit wear due to excessive bit bounce, bottomhole assembly whirl, bending of the BHA stick-slip phenomenon, torque, shocks, etc. Excessive values of such drill string parameters and other parameters relating to the drilling operations are referred to as dysfunctions. Many drill string and drill bit failures and other drilling problems can be prevented by properly monitoring the dynamic behavior of the bottom hole assembly and the drill bit while drilling and performing necessary corrections to the drilling parameters in real time. Such a process can significantly decrease the drilling assembly failures, thereby extending the drill string life and improving the overall drilling efficiency, including the rate of penetration.
Recently, patent application PCT/FR92/00730 disclosed the use of a device placed near the drill bit downhole for processing data from certain downhole sensors downhole to determine when the certain drilling malfunctions occur and to transmit such malfunctions uphole. The device processes the drilling data and compiles various diagnostics specific to the global or individual behaviors of the drilling tool, drill string, drilling fluid and communicates these diagnostics to the surface via the telemetry system. The downhole sensor data is processed by applying certain algorithms stored in the device for computing the malfunctions.
Presently, regardless of the type of the borehole being drilled, the operator continually reacts to the specific borehole parameters and performs drilling operations based on such information and the information about other downhole operating parameters, such as the bit bounce, weight on bit, drill string displacement, stall etc. to make decisions about the operator-controlled parameters. Thus, the operators base their drilling decisions upon the above-noted information and experience. Drilling boreholes in a virgin region requires greater preparation and understanding of the expected subsurface formations compared to a region where many boreholes have been successfully drilled. The drilling efficiency can be greatly improved if the operator can simulate the drilling activities for various types of formations. Additionally, further drilling efficiency can be gained by simulating the drilling behavior of the specific borehole to be drilled by the operator.
The present invention addresses the above-noted deficiencies and provides an automated closed-loop drilling system for drilling oilfield wellbores at enhanced rates of penetration and with extended life of downhole drilling assembly. The system includes a drill string having a drill bit, a plurality of sensors for providing signals relating to the drill string and formation parameters, and a downhole device which contains certain sensors, processes the sensor signals to determine dysfunctions relating to the drilling operations and transmits information about dysfunctions to a surface control unit. The surface control unit displays the severity of such dysfunctions, determines a corrective action required to alleviate such dysfunctions based on programmed instruction and then displays the required corrective action on a display for use by the operator.
The present invention also provides an interactive system which displays dynamic drilling parameters for a variety of subsurface formations and downhole operating conditions for a number of different drill string combinations and surface-controlled parameters. The system is adapted to allow an operator to simulate drilling conditions for different formations and drilling equipment combinations. This system displays the severity of dysfunctions as the operator is simulating the drilling conditions and displays corrective action for the operator to take to optimize drilling during such simulation.